This invention relates to a process for preparing fluid bed catalysts useful in the production of dicarboxylic acid anhydrides by the oxidation of hydrocarbons. More particularly it is directed to the preparation of fluid bed catalysts suitable for producing maleic anhydride from 4-carbon atom hydrocarbons, such as n-butane, n-butenes, 1,3 butadiene or a mixture thereof.
Catalysts containing vanadium and phosphorus oxides have been used in the oxidation of 4-carbon atom hydrocarbons, such as n-butane, n-butenes, 1,3 butadiene or mixtures thereof with molecular oxygen or oxygen-containing gas to produce maleic anhydride. Conventional methods of preparing these catalysts involve combining a vanadium compound, a phosphorus compound, and if desired, promoter element compound in a reducing medium under conditions which will provide vanadium in a valence state below +5 to form catalyst precursors capable of being converted to an oxide. The catalyst oxide precursor is then recovered and calcined to provide active catalytic material.
It has been taught in the art that various pentavalent and trivalent phosphorus compounds are satisfactory phosphorus components for use in the preparation of mixed vanadium phosphorus oxide catalysts. Orthophosphoric acid has been designated by some in the art as a preferred component.
U.S. Pat. No. 3,238,254 to Kerr lists the use of various phosphorus compounds such as metaphosphoric acid, triphosphoric acid, pyrophosphoric acid, orthophosphoric acid, phosphorus pentoxide, phosphorus oxyiodide, ethyl phosphate, methyl phosphate, amine phosphate, phosphorus pentachloride, phosphorus trichloride, phosphorus oxybromide, and the like, in the preparation of catalysts containing the mixed oxides of vanadium and phosphorus.
U.S. Pat. No. 3,474,041 to Kerr contains the above disclosure, and additionally discloses that vanadium phosphorus catalysts may be reactivated and stabilized by adding to the catalyst an organophosphorus compound.
U.S. Pat. No. 3,907,707 to Raffelson et al., 4,149,992 to Mount et al. and 4,179,404 to Barone disclose the preparation of vanadium phosphorus oxide catalysts using trivalent phosphorus compounds such as orthophosphorous acid, pyrophosphorous acid, methaphosphorous acid and hypophosphorous acid. Other phosphorus sources may include phosphorus trioxide and organic phosphites. A pentavalent phosphorus compound additionally could be utilized.
U.S. Pat. No. 4,013,586 to Dolan et al. discloses the preparation of vanadium phosphorus oxide catalysts using organo phosphonates as the source of phosphorus. A pentavalent phosphorus compound additionally could be utilized.
U.S. Pat. Nos. 3,888,886; 3,905,914; 3,931,046; 3,932,305 and 3,975,300 disclose the testing of promoted vanadium phosphorus oxide catalysts for maleic anhydride production from butane in one inch diameter fluid bed reactors. In most instances, the catalysts were prepared by forming the catalyst precursor in aqueous media (in U.S. Pat. No. 3,975,300 the precursor was formed in a paste of a vanadium compound, a phosphorus compound and an organic reducing agent), drying and thereafter grinding and sieving the precursor to a powder of about 74 to 250 microns size. This manner of preparation, however, does not obtain the uniform, microspheroidal catalyst particles preferred for successful fluid bed operation.
U.S. Pat. No. 4,043,943 discloses the preparation of the catalyst precursor in a liquid organic medium, preferably anhydrous, wherein the vanadium compound is reduced and solvated by gaseous HCl followed by reaction with the phosphorus compound.
The preparation of oxidation catalysts containing the mixed oxides of vanadium and phosphorus is disclosed in copending U.S. Ser. No. 106,786, assigned to our common assignee, now U.S. Pat. No. 4,244,879, wherein a vanadium compound is at least partially solubilized in an organic liquid medium capable of reducing at least a portion of the vanadium to a +4 valence state, and unsolubilized vanadium having a particle size larger than about 0.1 mm diameter is removed from the medium before addition of a phosphorus-containing compound.
The preparation of such catalysts is disclosed in copending U.S. Ser. No. 146,971, assigned to our common assignee, wherein partial reduction of a pentavalent vanadium compound is effected in the presence of a phosphorus compound in an organic liquid medium capable of reducing the vanadium.
It has been disclosed in copending U.S. Ser. No. 220,629 directed to the Preparation of Vanadium Phosphorus Oxide Catalysts Using a Mixed Phosphorus Source by P. R. Blum, E. C. Milberger, N. J. Bremer, D. E. Dria, and assigned to our common assignee, that vanadium phosphorus mixed oxide containing catalysts exhibit enhanced activity and selectivity to the production of maleic anhydride from 4-carbon atom hydrocarbons, such as n-butane, when such catalysts have been prepared from a mixed phosphorus source comprising orthophosphoric and pyrophosphoric acids. These catalysts are particularly effective when prepared in organic media.
As was discussed in Milberger et al., above, it had previously been found that contacting a vanadium phosphorus mixed oxide catalyst with water during preparation of the catalyst precursor resulted in diminution of surface area and activity of the catalyst for maleic anhydride production. In addition, contacting the calcined catalyst with water (below 100.degree. C.) reduces the activity of the catalyst.
Although excellent fixed bed catalyst disclosed in Blum et al., above, may be prepared without contacting the catalyst precursor with water, the method disclosed by Milberger et al., above, for the preparation of microspheroidal fluid bed catalyst particles dictates contacting the catalyst precursor with water. We initially found that vanadium phosphorus mixed oxide catalyst precursors which had been prepared using a mixed orthophosphoric/pyrophosphoric acid source in organic liquid media, had a tendency when contacted with water for slurrying prior to spray drying, to partially dissolve, forming a thick, viscous slurry unsuitable for spray drying.
It is therefore an object of the present invention to provide a process for the preparation of fluidizable, microspheroidal catalysts containing the mixed oxides of vanadium and phosphorus, which catalysts have been prepared from a mixed phosphorus source of orthophosphoric and pyrophosphoric acid.
It is a further object of the present invention to provide a process for producing maleic anhydride from 4-carbon atom hydrocarbons utilizing fluid bed vanadium phosphorus mixed oxide catalysts prepared using a mixed phosphorus source of orthophosphoric and pyrophosphoric acid.